Tachometer system with cylinder switch coupled to range switch



D C Voltage R. M. JORND TACHOMETER SYSTEM WITH CYLINDER SWITCHElectrical Input Circuit FIG. 1

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July 2. 1968 Filed Oct. 11, 1967 United States Patent 3,391,338TACHOMETER SYSTEM WITH CYLINDER SWITCH COUPLED TO RANGE SWITCH RussellM. Jornd, Dixon, Ill., assignor to Borg-Warner Corporation, Chicago,Ill., a corporation of Illinois Continuation-impart of application Ser.No. 396,835,

Sept. 16, 1964. This application Oct. 11, 1967, Ser.

8 Claims. (Cl. 324-70) ABSTRACT OF THE DISCLOSURE An electrical inputcircuit is coupled between a meter and the distributor breaker pointcircuit of an engine. The input circuit can be a unidirectional circuitto provide a DC voltage without employing a separate battery, or theinput circuit can include a switching arrangement for periodicallycharging a capacitor from a separate battery as a relay is actuated bythe distributor pulses. The voltage provided by the electrical inputcircuit is passed through a cylinder selection switch to aseries-coupled range switch, which is a multiganged switch for selectinglow, high and expanded scale readings. A battery is included in theexpanded scale circuit to produce a large meter deflection for a smallspeed variation when the expanded scale is selected.

BACKGROUND OF THE INVENTION This application is a continuation-in-partof an earlier application in the same name, originally entitled SpeedIndicating System, Ser. No. 396,835, filed Sept. 16, 1964, nowabandoned, and assigned to the same assignee.

The present invention is directed to a speed indicating systemespecially useful in conjunction with an internal combustion engine, andmore particularly to such a system which includes an electrical meterassociated with a novel shunting network which attords accurateindication over difierent speed ranges for engines with difierentnumbers of cylinders.

In the past electrical tachometer systems have been utilized to providea speed indication for an internal combustion engine. conventionally anelectrical meter, such as a milliammeter, has one side coupled both toan energy storage means and to an energy input circuit, to which abattery or other source of electrical energy may be coupled. Someswitching means, such as a relay or semiconductor unit, is provided andoperated by a signal derived from the distributor points to operate theswitching means at a rate which is a function of the engine speed. Therelay contacts or the semiconductor terminals can be connected tocomplete the circuit from the energy storage means first to the energyinput circuit and then to the meter. Thus the energy storage means,which may i be a capacitor, is alternatively charged from the inputcircuit and discharged through the meter to provide the speedindication.

There are various shortcomings in known speed indicating systems,especially in the lack of a system to accurately provide a speedindication for engines with dilferent numbers of cylinders, and toprovide such an indication over wide speed ranges. It is therefore aprimary consideration of this invention to provide a novel and unobvioustachometer type speed indicating system for use with engines withdifferent numbers of cylinders, and provide accurate indication of theengine operating speed over a wide range. Another salient considerationis the provision of an accurate tachometer system without employingswitching means, whether a relay, semiconductor unit or otherarrangement, for periodic operation as a function of the breaker pointpulses.

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In one embodiment the present invention includes an electrical metercoupled through an electrical input circuit to the breaker pointcircuit, to provide across the meter a DC voltage which is a function ofthe rate of operation of the breaker point circuit. In a preferredembodiment the electrical input circuit comprises passive componentswhich produce the desired D-C voltage of a given polarity for actuatingthe meter. In another embodiment a relay has its winding coupled to thebreaker point circuit and, in one position of the relay contacts acapacitor is charged from a battery with a polarity switch such that, inthe other position of the relay contacts, the charge accumulated in thecapacitor is passed through the meter to provide the desired speedindication.

In accordance with the present invention, a novel and unobvious variableshunting network is coupled in parallel with the meter. This networkincludes a first switching arrangement for selecting the range ofimpedance values to be placed in parallel with the meter as a functionof the number of cylinders in the engine having its speed monitored, anda second switching means is coupled in series with the first switchingarrangement to select a desired one of the impedance values within theselected range of values. Selection of this particular range ofimpedance values provides accurate operation of the system over low andhigh speed ranges, and even over an expanded scale arrangement in whicha large deflection of the pointer on the meter is produced responsive toa small variation in engine speed.

THE DRAWINGS The best mode contemplated for making and using theinvention will be described in connection with the accompanyingdrawings, in which like reference numerals identify like elements, andin which:

FIGURE 1 is a block diagram of the inventive arrangement;

FIGURE 2 is a schematic diagram depicting circuit details of a preferredembodiment of the invention; and

FIGURE 3 is a schematic'diagram illustrating another embodiment of theinvention.

GENERAL DESCRIPTION FIGURE 1 indicates an electrical meter 10 coupledover line 11 to an electrical input circuit 12. Pulses from the breakerpoint circuit are applied over line 13 to the electrical input circuit,wherein the pulses are rectified to produce a substantially constant D-Cvoltage for application over line 11 to the meter. In accordance withthe present invention a variable shunting network 16 is coupled overline 15 to the meter. The details of the electrical input circuit andthe variable shunting network will be explained in conjunction withFIGURES 2 and 3.

First embodiment, FIGURE 2 The pulses received from the breaker pointcircuit are applied to terminals 13a and 13b in FIGURE 2. Theseterminals correspond to the conductor 13 in FIGURE 1. An inductance 20and a resistor 21 are coupled in series between input terminal 13a andconductor 22. A first Zener diode 23 is coupled in parallel with acapacitor 24, and these two components are coupled between conductors 22and 25. A second resistor 26 is coupled between Movable contact 43 isconnected to reference terminal 61, and contact 43 is displaceable toengage one of the fixed contacts 44, 51 and 55 in the same contact set.This contact set is a part of the range selection switch 32 and thus(although not indicated on the drawing for the sake of clarity) movablecontact 43 is displaced concomitantly with actuation of movable contacts40, 41 and 42. Three difierent capacitors 65, 66 and 67 are respectivelycoupled between common conductor and fixed contacts 44, 51 and 55.

A first selector switch 31 has a movable contact 33 coupled to referenceconductor 17, and contact 33 is displaceable to engage one of the fixedcontacts 34, 35 and 36 in the same contact set. In accordance with thepresent invention, contact 34 is coupled over conductor 70 to movablecontact 40 of range selector switch 32; fixed contact 35 of the cylinderselection switch is coupled over conductor 71 to movable contact 41 ofswitch 32; and fixed contact 36 of the first switch 31 is coupled overconductor 72 to the movable contact 42 of second selector switch 32.Thus the cylinder selection switch is electrically coupled in serieswith the range selector switch so that actuation of movable contact 33selects a particular movable contact in one contact set of the secondselector switch, which in turn is displaceable to select one range of aplurality of impedance value ranges corresponding to diiferent scalesettings on electrical meter 10.

Fixed contacts 45, 46 and 47 of range selector switch 32 are all coupledto conductor 73. A battery 74, resistor 75 and a potentiometer 76 areall coupled in series between conductors 73 and 18. Fixed contact 48 iscoupled through a potentiometer 77 to conductor 78, which is coupled toreference conductor 18; contact 49 is coupled over another variableresistor 80 to conductor 78, and contact 50 is coupled throughpotentiometer 81 to conductor 78. Fixed contact 52 is coupled through apotentiometer 82 to conductor 83, which is coupled to referenceconductor 18; contact 53 is coupled through a variable resistor 84 toconductor 83, and contact 54 is coupled through a potentiometer 85 toconductor 83.

In operation movable contact 33 is first displaced to engage one of thefixed contacts 34, 35 and 36, thus selecting a circuit arrangementcorresponding to the number of cylinders in the engine under test.Switch 31 is shown in the 4-cylinder position, and the other fixedterminals 35 and 36 correspond to the 6-cylinder and 8-cylinderpositions, respectively. Scale selection switch 32 is then actuated todisplace movable contacts 40-43 simultaneously and thus select eitherthe expanded, high or low scale setting of the instrument. The circuitis shown in the expanded-scale setting. Movable contacts 40-43 whencontacting fixed contacts 48-51 provide the high scale circuit, and whencontacts 40-43 engage fixed contacts 52-55, the circuit is in thelow-scale position. The amount of capacitance provided between diode 60and conductor 25 is diiferent for each scale setting, thus varying theamount of electrical charge which is accumulated as the breaker pointpulses are received at terminals 13a, 13b, and correspondingly varyingthe discharge from the capacitor in the circuit through diode 63 andmeter 10 to indicate the motor speed. Capacitor 62 serves to minimizefluctuations of the pointer in meter 10.

Assuming the circuit is operating in the position shown, and somecertain deflection of the meter is achieved, the setting ofpotentiometer 76 can be varied to provide a discharge current frombattery 74 over contacts 45, 40, conductor 70, contacts 34, 33,conductor 17, meter 10, conductor 18, potentiometer 76, and resistor75to the other side of battery 74. By bucking or opposing the dischargecurrent from capacitor 65 the meter gives a large physical displacementof the indicator for a small increment of speed variation to provide amore accurate metering of the engine under test.

When switch 32 is displaced to another position, say the high scaleosition, then capacitor 66 i placed in the circuit through diode 63 andmeter 10. The shunting circuit then extends from the meter overconductor 17, contacts 33, 34, conductor 70, contacts 40, 48,potentiometer 77, and conductors 78, 18 to the other side of meter 10.Other settings and correspondingly similar operation of the circuit willbe understood from the foregoing description. In addition a table ofcircuit values found operable in the schematic arrangement of FIGURE 2is set out at the end of the specification.

Second embodiment, FIGURE 3 Another arrangement of the speed indicatingsystem, shown in FIGURE 3, includes a variation of the electrical inputcircuit and a modification of the difierent impedance connections of thescale selection switch 32. As there shown, one side of the meter 10 iscoupled to conductor 17 and the other side of the meter is coupled toconductors 18 and 111. In this embodiment the electrical input circuitincludes a pair of capacitors 120 and 121. A relay 112 has a winding122, a movable contact 123 and a pair of fixed contacts 124 and 125. Theelectrical input circuit further includes a battery 127 coupled betweenconductors and 126. The battery is not discharged unduly in theindicated position of the relay, for capacitors and 121 are paper typecapacitors, and the leakage current is of the order of microamperes. Itwill be apparent to those skilled in the art that other appropriatearrangements, such as an alternating current supply and a rectifiercircuit, could be utilized to apply an appropriate energizing potentialbetween conductors 115 and 126. Conductors 128 and 130, between whichrelay coil 122 is connected, may be coupled to the distributor pointcircuit to provide for sequential energization of relay 112 as afunction of distributor movement, which in turn depends on engine speed.

Variable shunting network 16 includes a first switching arrangement 31and second switching arrangement 32, connected in series as explained inconnection with FIGURE 2. In the showing of FIGURE 3 a first impedancemeans 156, shown as a simple resistor, is coupled between conductors 18and 71. Another impedance means 157 is coupled between conductors 18 and72. Between conductor 18 and contacts 45-47 (the expanded scaleterminals of switch 32) is coupled a series circuit including an energyinput circuit 158, 160 and a potentiometer 161 which includes anadjustable arm or contact 162. A battery 163 is indicated between theconductors of energy input circuit 158, 160, and the battery is depictedwith a polarity such as to buck or oppose the energy flow from battery127, when both batteries are in the circuit at the same time.

Coupled between conductor 18 and the high-scale contacts 48-50 isanother circuit including a potentiometer 164 having a movable arm 165,a first impedance means 166 coupled between arm and contact 48, a secondimpedance means or resistor 167 coupled between arm 165 and contact 49,and the arm or movable contact 165 is coupled directly over conductor168 to contact 50.

A similar arrangement is provided for low-scale contacts 52-54, with oneside of potentiometer 170 being coupled to conductor 18. Movable arm 171of potentiometer 170 is coupled over a first impedance means 172 tocontact 52, over a second impedance means 173 to contact 53, and iscoupled directly over conductor 174 to terminal or contact 54.

With conductors 128 and 130 coupled to the breaker point circuit of thedistributor, a reverse kick-back voltage in the range of from 20 toabout 200 volts is provided across the points with each ignition pulse,and the magnitude of this voltage is ample to energize winding 122 andoperate relay 112. Before the relay is actuated, movable contact 123engages contact 124 and provides an obvious charging path for capacitor120, and also for capacitor 121 when switch 32 is in the expanded-scaleposition as indicated. Considering current flow as passing from thepositive terminal of the battery through the external .circuit and backto the negative terminal, such current flows from battery 127 overconductors 126, 17, the parallel combination of capacitors 120 and 121,contacts 123, 124, and conductor 115 back to battery 127. Accordinglyduring the time period that relay 112 is not energized, capacitors 120,121 are charged toward the voltage level of battery 127.

As the reverse kick-back voltage is applied to winding 122, relay 112 isenergized and movable contact 123 engages contact 125. Accordingly thecapacitor combination 120, 121 commences to discharge over conductor 17,the internal resistance of meter 10, conductors 18 and 111, and contacts125, 123 back to capacitors 120, 121. The extent of the angulardisplacement of the indicator needle on meter is a function of theinternal meter resistance, the size" or electrical capacity of thecapacitors then in the discharge circuit, the rate at which relay 112 isenergized and deenergized, and the value of impedance or resistancecoupled in shunt with meter 10. The physical size of the capacitor andof the meter resistance being constant (although the capacitor size ischanged in different scale positions), the meter deflection is then afunction only of the rate of operation of relay 12, after switches 31and 32 in variable shunting network 16 have been positioned for theappropriate number of cylinders and the desired scale presentation onthe meter.

In the illustrated circuit diagram, switch 31 is in the 4-cylinderposition and switch 32 is in the expanded scale position. Accordingly,after relay 112 is energized, capacitors 120, 121 commence to dischargenot only through the meter but also the variable shunting circuit withinnetwork 16. This circuit extends from the meter over conductors 17,contacts 33, 34, conductor 70, contacts 40, 45, movable arm 162, theeffective portion of potentiometer 161, conductor 160, battery 163, andconductors 158, 18 back to the meter. Thus with potentiometer 161adjusted to accommodate production variations of the internal meterresistance and battery 163 being approximately equal to the voltage ofbattery 127, a large needle deflection on the meter is provided for asmall speed change of the engine.

With switch 31 in the 8-cylinder position (contact 33 engaging contact36) and switch 32 in the high scale position (contacts 40-43 engagingcontacts 48-51), the circuitry of the invention would provide a scalereading over a range of, by way of example, from 0 to 5000 revolutionsper minute. By providing switch 32, the expanded scale can be used atthe higher speeds and thus a much more accurate indication of the speedvariation at such speeds is obtained. Switch 31 adapts the noveltachometer circuit for use with engines having different numbers ofcylinders. Accordingly the various cylinder arrangements and meter scalepresentations are provided in a compact and inexpensive arrangement,without the requirement of complicated switching networks and powersupply circuitry previously utilized in this art.

For additional stabilization of meter indication, mechanical suppressionof the meter movement can be provided'or a capacitor, such as capacitor62 in FIGURE 2, can be coupled in parallel with the meter to dampen theneedle action.

To assist others in making and using the invention, a table of valuesfor the various components utilized in the circuits of FIGURES 2 and 3are set out below. It is emphasized, however, that this information isby way of illustration and assistance only and in no sense by way oflimitation.

Capacitor 62 50 microfarads, volts DC.

Capacitor 1.5 microfarads, 100 volts D.C. Capacitor 66 0.22 microfarad,100 volts D.C. Capacitor 67 2.0 microfarads, 100 volts D.C.'Potentiometers 77, 80,

81, 82, 84, 85 1000 ohms. Potentiometer 76 2000 ohms. Resistor 21 220ohms, /2 watt. Resistor 26 1000 ohms, V2 watt. Resistors 28, 75 1500ohms, /z watt. Diode 23 1N961A. Diode 27 1N957A. Diodes 60, 63 10D1.

FIGURE 3 Meter 10 50 microampere movement. Battery 27 1.5 volts D.C.Battery 63 1.5 volts D.C. Capacitor 20 5 microfarads. Capacitor 21 5microfarads. Potentiometer 61 0-1500 ohms. Potentiometer 64 075 ohms.Potentiometer 70 0-750 ohms. Resistor 56 3600 ohms. Resistor 57 1200ohms. Resistor 66 47 ohms. Resistor 67 16.9 ohms. Resistor 72 453 ohms.Resistor 73 178 ohms.

The present invention provides a tachometer arrangement which is bothextremely compact and highly versatile as contrasted to prior artdevices. Engine performance over a wide speed range, for example, from 0to 5000 revolutions per minute can be read by utilizing the high scaleposition of switch 32. With ordinary tachometers indicating the speed atthe high end of such a scale, it would not be possible to check engineoperation at such speeds. However, by use of the present invention, theexpanded scale can be utilized to indicate small changes in revolutionsper minute at a predetermined operating speed. Thus, after the engine isrun up to its normal operating speeed, the high voltage for each sparkplug circuit can be shorted to ground, and the engine speed shoulddecrease by the same value each time one of these circuits is'shorted.Accordingly the efficiency of the ignition high voltage cables, of thespark plugs themselves, and the compression ratio of the engine can bequickly and accurately checked.

Although only particular embodiments of the invention have beendescribed and illustrated, it is apparent that modifications andalterations may' be made therein. It is therefore the intention in theappended claims to cover all such modifications and alterations as mayfall within the true spirit and scope of the invention.

What is claimed is 5' a 1. In a speed indicating system for an engineunder test which includes a plurality of cylinders and a distributorhaving a breaker point circuit, the combination of an electrical meterhaving a pair of terminals,

an electrical input circuit coupled between the meter and the breakerpoint circuit to apply across the meter a DC voltage which is a functionof the rate of operation of the breaker point circuit, and

a. variable shunting network, coupled in parallel with said meter,including a first switch having a movable contact coupled to oneterminal of the electrical meter and having a preassigned number offixed contacts, each fixed contact representing one circuit of aplurality of electrical circuits related to the number of cylinders inan engine, and a second switch including a plurality of mutuallyinsulated moveable contacts each respectively coupled to one of thefixed contacts in the first switch, a plurality of fixed contact sets inthe second switch, each set of fixed contacts being positioned forengagement by one of the movable contacts in the second switch, eachfixed contact set representing a different meter scale setting, aplurality of variable impedance circuits each having an input connectionand an output connection, means individually coupling the inputconnection of each variable impedance circuit respectively to one fixedcontact in each of the fixed contact sets in the second switch, andmeans coupling the other terminal of the electrical meter to the outputconnec ion of each of the variable impedance circuits.

2. A speed indicating system as claimed in claim 1 in which said inputcircuit comprises a pair of terminals for coupling to the breaker pointcircuit, a Zener diode coupled in parallel with said terminals, a seriescircuit including a first diode and a capacitor coupled in parallel withsaid Zener diode, and a second diode coupled between said electricalmeter and the common connection between said first diode and thecapacitor, such that the charge accumulated in said capacitor uponreceipt of the pulses from the breaker point circuit is graduallydischarged through said second diode and the meter to provide anindication of the engine speed.

3. A speed indicating system as claimed in claim 1 in which saidelectrical input circuit comprises a pair of input terminals forcoupling to said breaker point circuit, a relay including a windingcoupled to said input terminals and further comprising a movable contactand a pair of fixed contacts for selective engagement by said movablecontact, a battery coupled between a first reference conductor and oneof said fixed relay contacts, a capacitor coupled between said onereference conductor and said movable relay contact, means electricallycoupling a second reference conductor to the other of said fixed relaycontacts and means coupling each of said meter terminals to a respectiveone of said reference conductors, such that receipt of successive pulsesat said input terminals effects successive relay operations whichalternatively charge said capacitor from the battery and provide acircuit for discharge of the capacitor through the meter to provide anindication of the engine speed.

4. In a speed indicating system for an engine under test which includesa plurality of cylinders and a distributor having a breaker pointcircuit, the combination of an electrical meter having a pair ofterminals,

an electrical input circuit coupled between the meter and the breakerpoint circuit to apply across the meter a D-C voltage of given polarityand an amplitude which is a function of the rate of operation of thebreaker point circuit, and

a variable shunting network, coupled in parallel with said meter,including a first switch having :a movable contact coupled to oneterminal of the electrical meter and having three fixed contacts, eachfixed contact representing one circuit of three electrical circuitsrelated to four-cylinder, six-cylinder and eight-cylinder engines, asecond switch including at least three contact sets each having amovable contact and three fixed contacts, the three fixed contacts ineach set representing the expanded scale, high scale and low scale meterpresentations to be selected by actuation of the second switch, meansrespectively coupling each movable contact of the second switch to oneof the fixed contacts in the first switch, three variable impedancecircuits providing three ranges of shunting impedance values forselection by the second switch to provide different meter scaleindications after the first switch provides cylinder selection, eachvariable impedance circuit having three input connections and a singleoutput connection, means individually coupling the three inputconnections of each variable impedance circuit to a respective one ofthe fixed contacts in each of the three contact sets in the secondswitch, and means coupling the other terminals of the electrical meterto the output connection of each of the variable impedance circuits.

5. A speed indicating system as claimed in claim 4 in which the variableimpedance circuit which provides expanded scale operation includes abattery connected in the proper sense to oppose said given polarity ofsaid D-C voltage, and a potentiometer connected in series with saidbattery :and adjustable to vary the extent of the bucking effect of thebattery, to obtain a large deflection on the electrical meter for asmall change in engine speed at a high operating speed.

6. A speed indicating system as claimed in claim 4 wherein saidelectrical input circuit includes a pair of input terminals for couplingto the breaker point circuit, a series circuit comprising an inductance,a first resistor and a second resistor :all coupled in series betweenone of said input terminals and one of the meter terminals, a firstZener diode and a first capacitor coupled in parallel with each otherand also coupled between the other of said input terminals and thecommon connection between said first and second resistors, a secondZener diode and a third resistor coupled in parallel with each other andalso coupled between said other input terminal and said one meterterminal, a third switch having a movable switch contact and a pluralityof fixed contacts, said movable switch contact disposed for actuationconcomitantly with the three movable contacts of said second switch, avariable capacitor means coupled between said other input terminal andsaid fixed contacts of said third switch, a first diode coupled betweensaid one meter terminal and said movable contact of said third switch,and a second diode coupled between the other of said meter terminals andthe common connection between said first diode and said movable contactof said third switch, such that the capacitor placed in the circuit bythe movable contact is charged by distributor pulses received at saidelectrical circuit input terminals and the accumulated charge isdischarged through said second diode and the meter to provide anindication of the engine speed.

7. A speed indicating system as claimed in claim 4 in which saidelectrical input circuit comprises a pair of input terminals forcoupling to said breaker point circuit, a relay including a windingcoupled to said input terminals and further comprising a movable contactand a pair of fixed contacts for selective engagement by said movablecontact, a battery coupled between a first reference conductor and oneof said fixed relay contacts, a capacitor coupled between said onereference conductor and said movable relay contact, means electricallycoupling second reference conductor to the other of said fixed relaycontacts and means coupling each of said meter terminals to a respectiveone of said reference conductors, such that receipt of successive pulsesat said input terminals effects successive relay operations whichalternatively charge said capacitor from the battery and provide acircuit for discharge of the capacitor through the meter to provide anindication of the engine speed.

8. A speed indicating system for an engine under test which includes aplurality of cylinders and a distributor having a breaker point circuit,including an electrical meter having a pair of terminals, an electricalinput circuit including a pair of input terminals for coupling to thebreaker point circuit, a series circuit including an inductance, a firstresistor and a second resistor all coupled between one of said inputterminals and one of said meter terminals, a first Zener diode and afirst capacitor coupled in parallel with each other and coupled betweenthe other of said input terminals and the common connection between saidfirst and secondresistors, a second Zener diode and a third resistorcoupled in parallel with each other and coupled between said other inputterminal and said one meter terminal, a contact set including a movablecontact and three fixed contacts, second, third and fourth capacitorseach of which has one side coupled to said other input terminal and theother side coupled to a respective one of said fixed contacts, a firstdiode coupled between said one meter terminal and said movable contact,a series circuit including a fifth capacitor and a second diode coupledin parallel with said first diode, and means for coupling the other ofsaid meter terminals to the common connection between said fifthcapacitor and said second diode, and a variable shunting network,coupled in parallel with said meter, including a first switch having amovable contact coupled to said one terminal of the electrical meter andhaving three fixed contacts, each fixed contact representing one circuitof three electrical circuits related to four-cylinder, sixcylinder andeight-cylinder engines, a second switch including at least three contactsets each having a movable contact and three fixed contacts, the threefixed contacts in each set representing the expanded scale, high scaleand low scale meter presentations to be selected by actuation of thesecond switch, means respectively coupling each movable contact of thesecond switch to a respective one of the fixed contacts in the firstswitch, a first variable impedance circuit including a battery and afirst potentiometer coupled between said other meter terminal and onefixed contact in each of said three contact sets, a second variableimpedance circuit including second, third and fourth otentiometers eachcoupled between said other meter terminal and a respective one of saidcontacts in each of said three contact sets, a third variable impedancecircuit including fifth, sixth and seventh otentiometers each coupledbetween said other meter terminal and a respective one of said contactsin each of said three contact sets, said movable contact in theelectrical input circuit being displaced concomitantly with the threemovable contacts in said second switch to select one of said second,third and fourth capacitors in accordance with the meter scaleindication and variable impedance circuit selected by actuation of saidsecond switch.

References Cited UNITED STATES PATENTS 2,513,668 7/1950 Parker 324-703,134,943 5/ 1964 Evenson 324-70 3,219,926 11/1965 Dion 324-70 3,275,9329/1966 Parmater 32470 3,283,247 11/1966 Beller 324-70 RUDOLPH V.ROLINEC, Primary Examiner.

25 M. J. LYNCH, Assistant Examiner.

